The present invention relates to a method and apparatus for cooling the turbine section of a gas turbine. More specifically, the present invention relates to a method and apparatus for steam cooling a turbine without the danger of over-heating the turbine if the steam supply is cut-off.
A gas turbine is typically comprised of a compressor section that produces compressed air that is subsequently heated by burning fuel in a combustor. The hot gas from the combustor is directed to a turbine section where the hot gas is used to drive a rotor shaft to produce power.
The turbine section typically employs a plurality of alternating rows of stationary vanes and rotating blades. Since the vanes and blades are exposed to the hot gas discharging from the combustor, cooling these components is of the utmost importance. Traditionally, cooling was accomplished by bleeding compressed air produced in the compressor and directing it to the turbine so that the cooling air bypassed the combustor. Depending on the application, the bleed air was sometimes cooled in an air-to-air cooler prior to introducing it into the turbine.
After introduction into the turbine, the cooling air flowed through radial passages formed in the airfoil portions of the vanes and blades. Typically, a number of small axial passages were formed inside the vane and blade airfoils that connected with one or more of the radial passages so that cooling air was directed over the surfaces of the airfoils, such as the leading and trailing edges or the suction and pressure surfaces. After the cooling air exited the vane or blade it entered and mixed with the hot gas flowing through the turbine section.
Unfortunately, this traditional cooling approach has a detrimental impact on the thermal efficiency of the gas turbine. Although the cooling air eventually mixes with the hot gas expanding in the turbine, since it bypasses the combustion process the work recovered from the expansion of the compressed cooling air is much less than that recovered from the expansion of the compressed air heated in the combustor. In fact, as a result of losses due to pressure drop and mechanical efficiency, the work recovered from the cooling air is less than that required to compress the air in the compressor. Thus, it would be desirable to cool the vanes and blades without the use of compressed air bled from the compressor.
One approach suggested in the past has been to replace the compressed air with steam generated from exhaust heat. Unfortunately, under this approach, any interruption of the steam supply could have disastrous results due to the rapidity with which the turbine section can over-heat. Although compressed air from the compressor could be used as a backup source of air, when the supply of such air to the turbine is conventionally regulated by means of an isolation valve, the danger exists that such valve may malfunction or not respond sufficiently quickly to prevent over-heating of the turbine. Moreover, in order to ensure that steam does not inadvertently enter the compressor and damage the components therein, the conventional approach also requires a check valve in the compressed air piping. Again, the existence of this valve creates the danger that a valve malfunction will deprive the turbine of cooling in the event of an interruption in the steam supply or permit steam to enter the compressor.
It is therefore desirable to provide an extremely reliable method and apparatus for utilizing steam to cool the turbine section of a gas turbine.